1. Field of the Invention
The present invention relates to an LED driving method and driving power source device for enhancement of the luminous intensity of LEDs.
2. Description of the Related Art
The LED is a device in which its luminous intensity, efficiency, life, etc. are optimized at a rated current. Where a current that is smaller than the rated current flows through the LED, its luminous intensity is low though its life and efficiency are increased. Conversely, where a current that is larger than the rated current flows through the LED, although the luminous intensity is high, the efficiency lowers to generate more heat and the life is shortened because it is easily affected by heat. The LED may be broken if an unduly large current is caused to flow through it to increase the luminous intensity; it should be avoided to cause a current that is lager than the rated current to flow through the LED.
Therefore, conventionally, because of the above properties of the LED, constant current circuits are mainly used as LED driving power source devices. The only method for increasing the luminous intensity of an LED light source is to increase the arrangement density of LEDs.
Reference 1, JP-A-2006-58909 discloses a technique for improving the display quality of an electrooptic device. However, this technique is directed to an electrooptic device which uses an electrooptic element whose luminous intensity is proportional to the drive current. Since the luminous intensity of the LED is not proportional to the drive current, it is difficult to increase the luminous intensity of the LED by using this technique.
The inventor of this application worked on the development of techniques for the luminous intensity enhancement of LEDs and filed the Japanese application corresponding to Patent Reference 2, JP-A-2010-40659 on the basis of results of that project. The technique of Patent Reference 2 employs series connections of many LEDs. However, the voltage consumed by the LED increases as the current flowing through it is increased. Therefore, if different currents flow through a series connection of LEDs, a larger difference occurs between the voltages developing across the series connection of LEDs as the number of LEDs is increased. As a result, if a voltage is set for a large reference current, a large amount of power is consumed uselessly by current limiting resistors etc. when the current is small and, in addition, the power consumed by the current limiting resistors etc. raises a problem of heat generation. As such, the technique of Patent Reference 2 which employs series connections of LEDs has a problem of a large power loss; it is expected that a product based on this technique will not be very useful.
In view of the above, the inventor continued the development of necessary means and devices to solve the problem of useless consumption of power on the basis of a technique of increasing the luminous intensity by changing the drive interval in a pulsed driving method and filed an application corresponding to Patent Reference 3, JP-A-2012-05431. This technique can solve the above-described problem almost completely. More specifically, when series connections of many LEDs are used, the problem of a voltage difference between a series connection of LEDs through which a rated forward current flows and a series connection of LEDs through which an allowable forward current flows is solved (the useless power consumption is minimized) by preparing two voltage sources having high efficiency in advance and switching between those two voltage sources. However, because of the use of the two voltage sources, this technique raises other problems, that is, the cost and the volume occupied are increased.
Patent Reference 1: JP-A-2006-058909
Patent Reference 2: JP-A-2010-040659
Patent Reference 3: JP-A-2012-054351
In the above circumstances, the present inventor conceived the present invention by continuing the development to enhance the luminous intensity enhancement of LEDs, that is, making a study diligently to solve the problem of useless consumption of power on the basis of the technique of increasing the luminous intensity by changing the drive interval in the pulsed driving method (the same technique as in Patent Reference 3).